The present invention relates to a control system for a continuously variable transmission.
As a conventional control system for a continuously variable transmission, there have been proposed two types, one being an open-loop type, the other being a feed-back type.
In FIG. 1, there is shown a conventional system for a continuously variable transmission of the open-loop type. Means 2 for determining operating position of shift actuator determines a signal which instructs an operating position of a shift actuator 6 based on various kinds of signals 4 representative of an operating state of a vehicle, and it outputs the signal to the shift actuator 6. In accordance with this signal, the shift actuator 6 adjusts an actual reduction ratio in a continuously variable transmission 8 to the state instructed.
However, according to the control system of the open-loop type wherein the operating position of the shift actuator is determined and the shift actuator is adjusted to this operating position, there is an error in reduction ratio owing to dispersion and looseness of mechanical parts interconnecting the shift actuator and the shfit control valve and hysteresis of the shift control valve.
In FIG. 2, a conventional control system for a continuously variable transmisson of the feed-back type is shown. This control system comprises a means 12 for determining a target reduction ratio based on various kinds of signals 10 representative of an operating state of a vehicle, a feed-back control means 14 for generating a shift command signal carrying instructions to establish the target reduction ratio determined by the means 12 for determining target reduction ratio, a shift actuator 16 operable on the shift command signal, a continuously variable transmission 18 where a reduction ratio is controlled in response to action of the shift actuator 16, and a means 20 for detecting an actual reduction ratio in the continuously variable transmission 18. The feed-back control means 14 is constructed as follows. A difference between the target reduction ratio generated by the means 12 for determining target reduction ratio and the actual reduction ratio generated by the means 20 for detecting actual reduction ratio is obtained by an arithmetic unit 14a, this difference is integrated at an integrator 14b and then this integrated value is multiplied with a predetermined integral control gain K1 at a multiplier 14c. On the other hand, this difference obtained at the arithmetic unit 14a is multiplied with a predetermined proportional control gain K2 at a multiplier 14d. The values obtained at the multipliers 14c and 14d are added to each other at an adder 14e and the result is output to the shift actuator 16.
However, according to this control system of the feed-back type, since the shift command signal is determined based only on the difference between the target reduction ratio and the actual reduction ratio, if large control gains (K1, K2) are set for the feed-back control, hunting takes place during rapid acceleration when the difference between the target reduction ratio and the actual reduction ratio is large, and on the contrary if small control gains are set, the response becomes poor.